1. Field of the Invention
The present invention relates to a thin film magnetic head for a hard disk drive and a magneto-resistance element used as a reproducing unit in the thin film magnetic head.
2. Description of the Related Art
As hard disk drivers become ever-denser, performance improvement in thin film magnetic heads is required. As a thin film magnetic head, there is widely used a complex thin film magnetic head made by laminating a reproducing head that has a magneto-resistance element (MR element) for reading and a recording head that has an inductive electromagnetic conversion element for writing. Currently, a magneto-resistance element that is operated by feeding a current in parallel with a film surface, which is called a spin-valve-type Giant Magnetoresistance (GMR) element, is widely used.
In the spin valve GMR element, an insulation layer, called a gap layer, is arranged between the upper and lower shield layers made of soft magnetic metal films. The recording density in the bit direction is determined by a gap (a reproduction gap interval) between the upper and lower shield layers.
As the recoding density becomes higher, there is a strong requirement that the shield gap and the track be narrowed. The element area is reduced by the narrowed tracks and the element height that is attendant thereon is shortened, however, in the conventional arrangement, because the heat radiation efficiency is lowered as the area is reduced, the reliability of operating current is restricted. To solve these problems, Japanese Patent Laid-Open No. 09-288807 (hereinafter, called Document 1) discloses a head structure in which the first shield film, the second shield film, and the magneto-resistance element are electrically connected in series and no insulation layer is arranged between the shield films. Such a structure is called a Current Perpendicular to the Plane (CPP) structure, and is an essential technique to attain a recoding density over 200 Gbits/in2. A brief explanation is given of a so-called “spin valve type” CPP-GMR element below.
The CPP-GMR element has a laminated structure that is formed on a suitable material having two layers of ferromagnetic films that are separated by a conductive nonmagnetic interlayer.
FIG. 1 is a cross-sectional view showing one structure example of a CPP-GMR element. As shown in FIG. 1, the representative laminated structure of the spin-valve-type CPP-GMR element includes lower electrode 122, antiferromagnetic film 124, first ferromagnetic film 126, nonmagnetic interlayer 128, second ferromagnetic film 130, and upper electrode 132. In FIG. 1, the uppermost layer is upper electrode 132 and the lowermost layer is lower electrode 122, and the upper electrode and the lower electrode are omitted in the laminated structure, which will be described later.
The magnetization direction of second ferromagnetic film 130 is vertical to the magnetization direction of first ferromagnetic film 126 when the externally-applied magnetic field is zero. The magnetization direction of first ferromagnetic film 126 is fixed by making antiferromagnetic film 124 adjacent thereto and by applying unidirectional anisotropy energy (also called “exchange bias” or “coupling magnetic field”) to first ferromagnetic film 126 by exchange coupling between antiferromagnetic film 124 and first ferromagnetic film 126. Therefore, first ferromagnetic film 126 is called a fixed (pinned) layer, and is hereinafter called the fixed layer or the pinned layer. On the other hand, a structure in which shield films and a magneto-resistance element are connected through metal, has the advantage that the heat radiation efficiency is improved and the operating current becomes large. Also, in this element, the resistance value becomes larger as the cross-sectional area of the element becomes smaller, and the resistance change amount is increased. In other words, this element is advantageously suitable for narrowing the track width.
A practical CPP-GMR element is disclosed in Journal of The Magnetics Society of Japan, vol. 25, pp. 807 to 810 (2001) (hereinafter, called Document 2). Document 2 discloses a so-called spin-valve-type CPP-GMR element, in which at least, a PdPtMn antiferromagnetic film, a fixed layer made of a CoFeB film of which the magnetization direction is fixed by this antiferromagnetic film, a Cu nonmagnetic interlayer, a free layer made of a CoFeB film of which the magnetization direction is freely changed in accordance with the external magnetic field, and an upper electrode are sequentially laminated on a lower electrode. Document 2 describes that a magnetic resistance change rate (hereinafter, called a MR ratio or a MR change rate) of the CPP-GMR element is about 1.16% (see Table 2 in Document 2) in the single spin valve structure. This MR change rate is insufficient in terms of output when practical use is considered. On the other hand, in the dual spin valve structure that can obtain a relatively high MR change rate, since each film thickness of both the two spin valves is large, that the film thickness of the spin valve is large does not meet the requirement that the lead gap be narrow.
As one technique of improving the MR change rate of the CPP-GMR element, Journal of The magnetics Society of Japan, Vol. 26, pp. 979 to 984 (2002) (hereinafter, called Document 3) discloses a current-narrowed-type CPP-GMR structure. This technique controls the flow of the sense current to optimize the effect of spin-dependent scattering according to the material of the spin valve. Document 3 describes that the MR ratio has become to about 3% in the single-spin-valve structure. However, this MR change rate is not yet sufficient in terms of output when practical use is considered.
It is considered that hindrance to improvement in the MR change rate of the CPP-GMR element is caused by low spin polarizability of the material that mainly includes Fe and Co, which is used for the first ferromagnetic film and the second ferromagnetic film. As one technique for obtaining a high-output (high MR ratio) CPP-GMR element, half-metal having spin polarizability P of 1 is used as a ferromagnetic film (a ferromagnetic film for the fixed layer or the free layer). The lager is the spin polarizability P, the lager is the MR ratio. The reason (mechanism) that the resistance change amount (ΔR) is large when polarizability P is large, can be considered as follows. When polarizability P is large, one of up-spin and down-spin has a high state density near Fermi energy (namely, the number of conductive electrons is large), whereas the other has a low state density near Fermi energy (namely, the number of conductive electrons is small). Therefore, there is a large difference between a mean free path of up-spin conductive electrons and that of down-spin conductive electrons, and the MR change rate is large. Incidentally, spin polarizability P of the metal ferromagnetic film for the free layer and the fixed layer, which is applied to the conventional CPP-GMR, is small, about 0.3 to 0.5 that is extremely small in comparison with the spin polarizability P of half-metal, which is 1.
The Journal of Magnetism and Magnetic Materials Vol. 198 and 199, pp 55 to 57 (1999) (hereinafter, called Document 4) discloses a technique of a CPP-GMR using one kind of half-metal, called Huesler alloy. The MR change rate is not so large, about 8% at 4.2K, however, it is meaningful since Document 4 suggests that Huesler alloy is available to the CPP-GMR element.
Now, a brief explanation is given of Huesler alloy. Huesler alloy is a general term of intermetallic compounds having a chemical composition of XYZ or X2YZ. The former is called half-Huesler and the latter is called full-Huesler. In this description, X is a transition metal element of the Co, Ni, or Cu group or precious metal in the periodic table. Y is a transition metal of the Mn, V or Ti group. Z is a typical element from group III to group V. Huesler alloy is named after F. Heusler who found that Cu2MnAl alloy, which does not include any ferromagnetic element, shows ferromagnetism. It is considered that the ferromagnetism that can be produced by magnetic moments are regularly arranged, i.e., by RKKY interaction. Applied Physics Letters Vol 79, Number 26, pp 4396 to 4398 (2001) (hereinafter, called Document 5) experimentally validates that Co2MnSi, which is full-Heusler alloy, has a highly-saturated magnetization of about 5μB and discusses that a high spin polarizability P and a high MR change rate, when used for the magneto-resistance element, are expected.
Japanese Patent Laid-Open No. 2003-218428 (hereinafter, called Document 6) discloses that X2YZ, in particular, Co2MnZ (Z═Al, Si, Ga, Ge, Sn) is used for the CPP-GMR element.
Japanese Patent Laid-Open No. 2004-221526 (hereinafter, called Document 7) discloses that Co2(FexCr1-x)Al is used for the TMR element or for the CPP-GMR element.
Physical Review B66, 174429 (2002) (hereinafter, called Document 8) discloses the calculated values of the spin moment in Heusler alloy, such as RuMnZ and RhMnZ, in table IV. The calculated values are shown in FIG. 2.
When X disclosed in Document 8 is used as the CPP-GMR element since a high spin moment can be obtained by full-Heusler alloy formed of precious metal, such as Ru, Rh, Ir, Pd, Pt, a high MR change rate caused by the high bulk diffusion effect can be expected. On the other hand, as described in Magnetic Material Handbook (ISBN4-254-13004-X), Asakura Publishing Co., Ltd., p374, there is a problem that the full-Heusler alloy formed of precious metal, such as Ru, Rh, Ir, Pd, Pt, generally has a low Curie temperature and is unsuitable for application to heads.